Multiple personality, or multiple personality disorders (MPD) is when a person is displaying two or more seperate personalities.
These personalities often go by different names and the stronger new/stronger personality is often aware of the orignal/others whereas the original personality often just recalls 'blacking out'.

Psychologists Thigpen and Cleckley conducted a study on a woman claiming to have 'blinding headaches', which concluded she infact was one of the vary rare cases of reported MPD victims. (For further reading, this study can be found here: http://www.holah.co.uk/, and is explained well in the film based on this study 'The Three Faces Of Eve').

(Taken from Thigpen and Cleckley's study on multiple personalities)

During the interview, Eve White who was normally very self-controlled became distressed and asked whether hearing an occasional imaginary voice made her insane.

She reported that she had on several occasions over the last few months briefly heard a voice addressing her. During this conversation Eve White, as if in pain suddenly put both hands to her head. After a tense moment of silence her hands dropped, and the therapist observed a ‘quick, reckless smile’ and in a bright voice she said: ‘Hi there, Doc’